The invention is based on the so-called fluidized bed jet milling process, in which a jet of gas or steam exiting a nozzle at high speed is introduced into a fluidized bed of granular material. The particles in the vicinity of the jet are accelerated to such a high speed that they shatter upon impact against still or approaching particles. Such a process--especially one suitable for fine grinding--is disclosed in U.S. Pat. No. 1,948,609.
A disadvantage of the process just mentioned, however, is that the kinetic energy introduced by the jet is only partially used for the actual comminution. The pressure drop in the jet with respect to the material bed causes particles to be immediately drawn into the jet and accelerated. However, such a change of momentum only takes place in the outermost peripheral zone of the jet. The core area of the jet remains practically free of product, so that the kinetic jet energy in this zone is largely unused, resulting in unsatisfactory comminution efficiency.
U.S. Pat. No. 3,734,413 describes an apparatus for forcing the product laterally into the jet by mechanical conveying means. This apparatus, however, requires a considerable amount of mechanical equipment and high power consumption. Also, the conveying equipment is subject to a high degree of wear. The same disadvantages are displayed by the well-known injector jet mills, e.g. as per U.S. Pat. No. 1,935,344 where the product is mixed with the gas or steam in an acceleration nozzle before the jet is formed.